In modern assembly techniques for joining metal components in machine construction, vehicle or equipment manufacture, more especially in car manufacture, conventional methods of fixing, such as riveting, screwing or welding, are being increasingly replaced by bonding. Spot welding above all, which is a source of future corrosion, is being displaced as far as possible or is being applied in combination with structural adhesives. For this reason, there is an increased demand for high-strength structural adhesives. For assembly reasons, these adhesives have to be used at the so-called white-shell stage of car manufacture, i.e. the adhesives are generally applied to the uncleaned metal surface. These surfaces are often coated with various corrosion-inhibiting oils and drawing oils, so that the adhesives used there should not be functionally affected by these oils. In addition, the adhesives should be capable of withstanding--preferably without pregelation--the various washing baths and installations and the high temperatures of up to around 240.degree. C. prevailing in the baking ovens for electrocoating and should also cure at temperatures of that order. Moreover, the adhesives are required to exhibit good ageing-resistant adhesion to various galvanized steels, for example electrolytically galvanized steel plates, hot-dip galvanized steel plates and the corresponding galvannealed steel plates or galvanized and subsequently phosphated steel plates. Structural adhesives for these applications must also have a minimum strength of about 15 MPa. In the interests of smooth assembly line operation, only one-component materials capable of being transported by pumps and applied by machine are suitable.
On account of the demanding strength requirements, one-component hot-curing epoxy adhesives have mainly been used for these applications in the past. Apart from the advantages of high tensile strength, however, epoxy adhesives have a number of major disadvantages. The paste-like, hot-curing one-component epoxy adhesives do not show adequate resistance to washing in the washing and phosphating baths, so that the corresponding bonds normally have to be pregelled by induction heating or in special ovens. Unfortunately, this involves an additional step. Attempts have been made to overcome this by developing one-component hot-curing epoxy adhesives resembling hotmelts in character. Unfortunately, these adhesives require special application systems because they have to be applied hot. Another general disadvantage of epoxy adhesives is their tendency to absorb moisture under the effect of high atmospheric humidity which can lead to corrosion phenomena and weakening of the bond in the bond line. Although epoxy adhesives are distinguished by high tensile strength, their breaking elongation is generally very poor; even epoxy adhesives flexibilized by addition of rubber have a breaking elongation of less than 5%. In addition, the use of epoxy adhesives based on low molecular weight epoxy compounds (molecular weight&lt;700) is undesirable on industrial hygiene grounds because these low molecular weight epoxy compounds can initiate allergic or sensitizing reactions on contact with the skin.
For some time, compositions based on vulcanizable rubbers have been used as an alternative. EP-B-97 394 describes an adhesive mixture based on a liquid polybutadiene rubber, powder-form sulfur, organic accelerators and optionally solid rubber. According to B. D. Ludbrook, Int. J. Adhesion and Adhesives, Vol. 4, No. 4, pages 148-150, corresponding adhesives based on liquid polybutadienes are capable of attaining strength levels equivalent to those of flexibilized epoxy adhesives through an appropriate choice of the quantity of sulfur and accelerators. Whereas these formulations have good curing properties and show high resistance to ageing and even adhere acceptably to normal oiled steel plate, their usefulness for various galvanized steel plates is limited, in addition to which the breaking elongation of these high-strength rubber adhesives is very poor.
To improve adhesion, DE-C-38 34 818 proposes using OH-terminated polybutadienes for the liquid rubber. According to EP-B-441 244, homopolymers or copolymers containing thiol, amino, amido, carboxyl, epoxy, isocyanate, anhydride or acetoxy groups may be used in addition to hydroxyfunctional homopolymers or copolymers as the functional rubber polymer, although the cured adhesive mixture has a breaking elongation of no more than 15%.
According to EP-B-309 903 and DE-C-40 27 064, polyfunctional epoxy compounds may be added to the adhesive mixtures based on liquid rubbers to improved adhesion and tensile shear strength. Apart from the fact that it is undesirable for the reasons explained above to use adhesive compositions containing epoxy resin, the adhesive compositions disclosed in the last two documents are not suitable as structural adhesives because they only reach a very low strength level of at most 3 MPa.